The IEEE's standard for wireless LANs, designated IEEE 802.11, provides two different ways to configure a network: ad-hoc and infrastructure. In an ad-hoc network, computers form a network “on the fly,” with each computer or 802.11 device joining the network as is able to send and receive signals. There is no defined structure in an ad-hoc network; there are no fixed points; and every node in the network is able to communicate with every other node in the network. Although it may seem that order would be difficult to maintain in this type of network, sufficient algorithms, such as the spokesman election algorithm (SEA), are provided and are designed to “elect” one machine as the base, or master, station of the network, with the others machines being “slaves.” Another algorithm in ad-hoc network architectures uses a broadcast and flooding method to all other nodes to establish the identity of all nodes in the network.
The infrastructure architecture provides fixed network access points for communications with mobile nodes. These network access points (APs) are sometime connected to land lines to widen the LAN's capability by bridging wireless nodes to other wired nodes. If service areas overlap, handoffs may occur between wireless LANs. This structure is very similar to that used in cellular networks.
IEEE 802.11 standard places specifications on the parameters of both the physical (PHY) and medium access control (MAC) layers of the network. The PHY layer, which actually handles the transmission of data between nodes, may use either direct sequence spread spectrum, frequency-hopping spread spectrum, or infrared (IR) pulse position modulation. IEEE 802.11 makes provisions for data rates of up to 11 Mbps, and requires operation in the 2.4–2.4835 GHz frequency band, in the case of spread-spectrum transmission, which is an unlicensed band for industrial, scientific, and medical (ISM) applications; and in the 300–428,000 GHz frequency band for IR transmission. Infrared is generally considered to be more secure to eavesdropping, because IR transmissions require absolute line-of-sight links, i.e., no transmission is possible outside any simply connected space or around corners, as opposed to radio frequency transmissions, which can penetrate walls and be intercepted by third parties unknowingly. However, infrared transmissions can be adversely affected by sunlight, and the spread-spectrum protocol of 802.11 does provide some rudimentary security for typical data transfers. Additionally, IEEE 802.11a allows for transmission in the 5 GHz UNII bands, 5.15 GHz to 5.25 GHz and 5.25 GHz to 5.356 GHz, at data rates up to 54 Mbps.
The MAC layer includes a set of protocols which is responsible for maintaining order in the use of a shared medium. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet to be transmitted, it first listens to ensure no other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise, it chooses a random “backoff factor,” which determines the amount of time the node must wait until it is allowed to transmit its packet. During periods in which the channel is clear, the transmitting node decrements its backoff counter. When the channel is busy it does not decrement its backoff counter. When the backoff counter reaches zero, the node transmits the packet. Because the probability that two nodes will choose the same backoff factor is small, collisions between packets are minimized. Collision detection, as is employed in Ethernet®, cannot be used for the radio frequency transmissions of IEEE 802.11, because when a node is transmitting, it cannot hear any other node in the system which may be transmitting, because its own signal will block any other signals arriving at the node. Whenever a packet is to be transmitted, the transmitting node first sends out a short ready-to-send (RTS) packet containing information on the length of the packet. If the receiving node hears the RTS, it responds with a short clear-to-send (CTS) packet. After this exchange, the transmitting node sends its packet. When the packet is received successfully, as determined by a cyclic redundancy check (CRC), the receiving node transmits an acknowledgment (ACK) packet. This back-and-forth exchange is necessary to avoid the “hidden node” problem, i.e., node A can communicate with node B, and node B can communicate with node C. However, node A cannot communicate node C. Thus, for instance, although node A may sense the channel to be clear, node C may in fact be transmitting to node B. The protocol described above alerts node A that node B is busy, and requires node A to wait before transmitting its packet.
Although 802.11 provides a reliable means of wireless data transfer, some improvements to it have been proposed. The use of wireless LANs is expected to increase dramatically in the future as businesses discover the enhanced productivity and the increased mobility that wireless communications can provide.
The IEEE Standard P802.11a, July, 1999 (Supplement to IEEE Std. 802.11-1999), provides background for this invention. Referring to FIG. 1, the 802.11a PHY layer transmit chain is depicted generally at 10. Chain 10 includes a convolution encoder 12, and interleaver/construction mapper 14, an OFDM modulator 16, a symbol shaper 18, an unconverter/transmitter 20, and an antenna 22. Currently, 802.11a uses only convolutional encoder 12 to provide forward error correction (FEC). The Quality of Service (QoS) prior art for IEEE 802.11a uses the automated response query (ARQ) mechanism, which imposes longer delay and jitter, and can impede the enforcement of QoS policies. These QoS enhancements will enable the transmission of high quality video, e.g., MPEG2, which requires data rates up to 24 Mbps. However, for consumer audio-video equipment, and particularly for interactive applications, such as remote control of high-quality audio-video, both high bandwidth and low latency transmission are required. Thus, these applications demand a minimal use of the ARQ mechanisms. One solution to this problem is to provided FEC, or in the case of 802.11a, additional FEC. International Standard CEI/IEC 61883-4, Consumer audio/video equipment digital interface, Part 4: MPEG2-TS data Transmission; First Edition, February 1998.
There is additional FEC proposed for 802.11b in the Joint Proposal for QoS Enhancements, Submitted by AT&T, Sharewave, Lucent, et al., presented Sep. 18, 2000 in IEEE 802.11, and listed as document number 802.11-00/120r1 to IEEE 802.11-1999 standard, however, because its modulation scheme is different from the basic scheme of 802.11, and because its data rates are lower, the Joint Proposal scheme, if used in 802.11a, will impose significant reductions in channel efficiency due to having to transmit parity data for a large payload when oftentimes there is a small payload, such as is the case with many management frames. In the Joint Proposal to 802.11e, additional coding is proposed for high quality transmission. While such a coding scheme was envisaged for 802.11b, that coding scheme is not adequate to work with the 802.11a physical layer requirements.
The use of an outer code in a wireless communication system to provide additional FEC has been proposed in Lin and Costello, Error Control Coding: Fundamentals and Applications, Prentice-Hall, 1983, and in R. McEliece, The Theory of Information and Coding, Addison-Wesley, 1977. However, because of the legacy of 802.11 as an ARQ based system, there is little prior art in this field except for the block coding scheme of the Joint Proposal, which that code is for a complementary code keying/packet binary code keying (CCK/PBCK) modulation system, which has interframe spacing different from that of 802.11a. A different coding methodology is required for 802.11a, which codes data in multiples of 24 data symbols. The proposed CCK/PBCK coding methodology is not applicable to 802.11a, or other CSMA/OFDM wireless networks.
In the IEEE 802.11a standard (1999), the channel link is designed, for various received signal levels, to maintain a 90% packet throughput in the absence of collisions. If a packet is successfully received, an ACK is sent to the sending station (STA). If an ACK is not received, after a suitable time, the packet is re-sent. The loss of packets in 802.11 systems adds considerable latency to the system, as well as jitter, i.e., the packet delivery can vary with time.
Moreover, it is desirable to be able to transmit high quality AV over 802.11a, one of two competing 5 GHz wireless LAN technologies, the other being HiperLAN 2, in the home network. A wireless home network, combined with device discovery features, will enable the home network user to rapidly deploy his network with a minimum of effort. Because of the anticipated earlier market introduction of 802.11a systems, 802.11a is a leading, if not the leading, candidate for home network deployment in the 5 GHz band. However, in order for 802.11a to be successful in this market area, a reliable, low jitter, low latency mode is needed, which mode is not currently provided in the standard. A goal of the invention is to provide less than one frame error in 10 hours of transmission, with at most, one packet retry, assuming the link conditions are met for 802.11a.
U.S. Pat. No. 5,949,796 to Kumar, granted Sep. 7, 1999, for In-band on-channel digital broadcasting method and system, describes an in-band, on-channel digital signal for FM RF broadcasts.
U.S. Pat. No. 5,475,716, granted Dec. 12, 1995, to Huang, for Method for communicating block coded digital data with associated synchronization/control data, describes blocked coded digital data transmission, but does not describe a CSMA transmission using an OFDM PHY.